The present invention relates to a piercing method for the manufacture of seamless metal pipe from a polygonal billet.
The conventional piercing methods for making seamless metal pipe (for example, seamless steel pipe) include the press piercing method and the inclined roll piercing method, and, recently, there has also come under consideration a press roll piercing method for piercing low priced square or rectangular billets into hollow shells in a single operation. The press roll piercing method is a technique of feeding a square billet in its axial direction into a pair of driven rolling rolls mounted one above the other and of piercing the core portion of the billet while rolling the billet into a round shape, whereby the hollow shell is formed.
In designing and manufacturing a press roll piercing mill, it is necessary to determine what strength and structure the piercing mill proper should have and in this the rolling load is an important factor. Further, in determining the capacity required of the motor for driving the rolls and the strength and structure of the drive power transmitting system, an accurate estimation of the roll torque is required. However, no one has yet proposed a reliable formula for calculating these values. Factors making theoretical calculation difficult are that the press roll piercing method has only recently begun to be studied with a view to practical use and that it is not completely clear what unique deformation conditions occur during the process of piercing the square billet while it is being simultaneously pushed into a circular groove and rolled on its external surface. As a result, the technique of operating the press roll piercing mill is still not well established.